An electrode material used for an electrode of a vacuum interrupter (VI) etc. is required to fulfill the properties of: (1) a great current-interrupting capacity; (2) a high withstand voltage capability; (3) a low contact resistance; (4) a good welding resistance; (5) a lower consumption of a contact point; (6) a small interrupting current; (7) an excellent workability; (8) a great mechanical strength; and the like.
A copper (Cu)-chromium (Cr) electrode has the properties of a good current-interrupting capacity, a high withstand voltage capability, a good welding resistance and the like and has widely been used as a material for a contact point of a vacuum interrupter. The Cu—Cr electrode has been reported that Cr particles having a finer particle diameter are more advantageous in terms of the current-interrupting capacity and the contact resistance (for example, by Non-Patent Document 1).
As a method for producing a Cu—Cr electrode material, there are generally two well-known methods, i.e. a sintering method (a solid phase sintering method) and a infiltration method. In the sintering method, Cu having a good conductivity and Cr having an excellent arc resistance are mixed at a certain ratio, and the mixed powder is press-molded and then sintered in a non-oxidizing atmosphere (for example, in a vacuum atmosphere), thereby producing a sintered body. The sintering method has the advantage that the composition between Cu and Cr can freely be selected, but it is higher in gas content than the infiltration method and therefore has a fear of being inferior to the infiltration method in mechanical strength.
On the other hand, in the infiltration method, a Cr powder is press-molded (or not molded) and charged into a container and then heated to temperatures of not lower than the melting point of Cu in a non-oxidizing atmosphere (for example, in a vacuum atmosphere) to infiltrate Cu into airspaces defined among Cr particles, thereby producing an electrode. Although the composition ratio between Cu and Cr cannot freely be selected, the infiltration method has the advantage that a material smaller than the sintering method in gas content and the number of airspaces is obtained, the material being superior to the sintering method in mechanical strength.
In recent years, conditions for the use of the vacuum interrupter are getting restricted while the application of the vacuum interrupter to a capacitor circuit is increasingly developed. In a capacitor circuit, a voltage which is twice or three times larger than the usual one is applied between electrodes, so that it is assumed that a surface of a contact point receives significant damages by arc generated at current-interrupting time or current-starting time, thereby causing reignition of arc easily. For example, when closing electrodes under a state of applying circuit voltage, an electric field between a movable electrode and a fixed electrode is so strengthened as to cause an insulation breakdown before the electrodes are closed. An arc is to be generated at this time, and the heat of the arc cause melting in the surfaces of the contact points of the electrodes. After the electrodes have been closed, the melted portions are reduced in temperature by thermal diffusion so as to be welded. When opening the electrodes, the welded portions are stripped from each other and therefore the surfaces of the contact points are to be damaged. Hence, there has been desired an electrode material having better withstand voltage capability and current-interrupting capability than those of the conventional Cu—Cr electrode.
As a method for producing a Cu—Cr based electrode material excellent in electrical characteristics such as withstand voltage capability and current-interrupting capability, there is a method of producing an electrode where a Cr powder for improving the electrical characteristics and a heat resistant element powder (molybdenum (Mo), tungsten (W), niobium (Nb), tantalum (Ta), vanadium (V), zirconium (Zr) etc.) for refining the Cr powder are mixed into a Cu powder as a base material and then the mixed powder is charged into a mold and press-molded in order finally to obtain a sintered body (Patent Documents 1 and 2, for example).
To be more specific, a heat resistant element is added to a Cu—Cr based electrode material originated from Cr having a particle diameter of 200-300 μm, thereby refining Cr through a microstructure technique. That is, an alloying of Cr and the heat resistant element is accelerated, and thereby increasing deposition of fine Cr—X particles (where X is a heat resistant element) in the interior of the Cu base material structure. As a result, Cr particles having a particle diameter of 20-60 μm is uniformly dispersed in the Cu base material structure, in the form of including the heat resistant element in the interior thereof.
In order to improve an electrode material in electrical characteristic such as current-interrupting capability and withstand voltage capability, it is required that a content of Cr and that of a heat resistant element are large in the Cr base material and that Cr and particles where Cr and the heat resistant element are changed into a solid solution are miniaturized in particle diameter and then uniformly dispersed in the Cu base material.